As someone who has watched all of Supernatural and continues to watch it, my best advice is STAY AWAY. If you cannot, I recommend seasons 3-5. Seasons 1 and 2 are all right, though filled with monster-of-the-week episodes that might be annoying.
Well, since I ended up sleeping pretty much the entire day, knocked out on pain killers, I only managed to watch half an episode of MI-5. Supernatural will have to stay unwatched, for now. Probably for the best.